1. Field of the Invention
The present invention relates to a light emitting element driving circuit which can stably control, even at a low voltage supply voltage, an amount of output light of a light emitting element to be used in an image formation device such as a laser printer or the like.
2. Description of the Related Art
An image formation device is the device which converts an electrical signal into a light signal, and writes an image by using light based on the converted light signal. Here, in the image formation device, to drive a light emitting element such as a laser diode for converting the electrical signal into the light signal, a light emitting element driving circuit for supplying a current to the light emitting element according to an image signal is used.
FIG. 7 is a block diagram illustrating an example of the constitution of a conventional light emitting element driving circuit. To make an amount of light of a laser diode 101 acting as the light emitting element constant, the light emitting element driving circuit of this type is equipped with an APC (automatic power control), which executes an automatic light-amount adjustment function, to monitor the amount of light of the laser diode 101 by using a photodiode 102 acting as a light detecting element. Here, the photodiode 102 monitors the amount of light of the light emitting element 101, and executes photoelectrical conversion thereof to generate a first current I11.
To acquire a desired light emitting element driving current, a driving current adjustment circuit 103 controls the gate voltage of an NMOS (Negative-channel Metal Oxide Semiconductor) transistor 104 to be used for determining a driving current at a voltage correlated with the first current I11, according to a control voltage V11. A second current I12 correlated with the first driving current I11 flows in the drain of the NMOS transistor 104. A differential switch circuit which is constituted by an NMOS transistor 105 and an NMOS transistor 106 is controlled in response to image signals V12 and V13. Then, the differential switch circuit executes switching between the laser diode 101 and a resistor 107 to flow the second current I12, thereby modulating the driving current of the laser diode 101.
In recent years, the power supply voltage of a commonly used system has reduced from 5V to 3V. Moreover, to simplify the system and reduce costs by downsizing power supply IC's, it is required to set the power supply voltage of the light emitting element driving circuit to 3V which is the same as the power supply voltage of the system.
However, as illustrated in FIG. 8, a power supply voltage Vcc has to supply various kinds of voltages for the light emitting element driving circuit of FIG. 7. More specifically, the power supply voltage Vcc supplies a forward voltage Vld of the laser diode 101, a source-drain voltage Vds1 for driving the NMOS transistor 104, and a source-drain voltage Vds2 for driving the NMOS transistors 105 and 106. Here, it should be noted that the NMOS transistors 105 and 106 together constitute the differential switch circuit.
For example, it is assumed that the power supply voltage Vcc is 3V, and the forward voltage Vld is 2.3V. In the circumstances, the voltage which can be allocated to the source-drain voltages Vds of the two NMOS transistors is 0.7V which is a difference voltage between the power supply voltage Vcc and the forward voltage Vld. Consequently, if an NMOS transistor having a sufficiently large W/L (gate width/gate length) is not used, it is impossible to operate the NMOS transistor in a saturation region. For this reason, if the NMOS transistor having the sufficiently large W/L is not used, the drain current becomes unstable, and thus the APC operation is disturbed and the waveform of the light emitting element driving current is distorted. Therefore, in the circuit configuration illustrated in FIG. 7, it is difficult to acquire the stable amount of light at the power supply voltage 3V or so which is lower than the conventional general voltage 5V, whereby it is necessary to enlarge the W/L to acquire the stable amount of light.
FIGS. 9 and 10 respectively illustrate examples of circuits to solve such a problem as described above. More specifically, as well as the circuit illustrated in FIG. 7, the circuit illustrated in FIG. 9 controls the gate voltage of a PMOS (Positive-channel Metal Oxide Semiconductor) transistor 108 to be used for determining the driving current at the output voltage of the driving current adjustment circuit 103. A second current I2, which is correlated with a first current I1 input from the photodiode 102 to the driving current adjustment circuit 103, flows in the drain of the PMOS transistor 108. A differential switch circuit which is constituted by a PMOS transistor 109 and a PMOS transistor 110 is controlled in response to image signals V14 and V15, thereby executing switching of the second current I2. Thus, when the second current I2 flows in the PMOS transistor 109, the laser diode 101 is driven through a current mirror circuit which is constituted by an NMOS transistor 111 and an NMOS transistor 112. On the other hand, when the second current I2 flows in the PMOS transistor 110, the driving current of the laser diode 101 is modulated by flowing the second current I2 to the ground potential through an NMOS transistor 113.
In the above circuit constitution, the element to be serially connected to the light emitting element between the power supply and the ground potential is only the NMOS transistor 112. Thus, it is possible to supply the source-drain voltage which is sufficient to operate the NMOS transistor 109 in the saturation region even at the power supply voltage 3V or so, whereby it is possible to acquire the stable amount of light.
In the circuit in FIG. 10, an NMOS transistor 116 acting as a voltage resetting single-phase switch is connected between the gate of a current mirror circuit constituted by an NMOS transistor 114 and an NMOS transistor 115 and the ground potential. As well as the circuit illustrated in FIG. 9, the circuit illustrated in FIG. 10 inputs the determined second current I2 to the gate of the current mirror circuit, and controls the gate voltage of the NMOS transistor 116 in response to an image signal V16. Thus, it is possible to control the gate voltage potential of the current mirror circuit. Also, it is possible to modulate the driving current of the laser diode 101.
In this circuit constitution, as well as the circuit illustrated in FIG. 9, the element which is connected in series to the laser diode 101 between the power supply and the ground potential is set to only the NMOS transistor 115. Therefore, it is possible to acquire the stable amount of light at the power supply voltage 3V or so. It should be noted that the detail of this circuit configuration is disclosed in Japanese Patent Application Laid-Open No. H11-126935.
However, in the light emitting element driving circuit illustrated in FIG. 9, the switched current is supplied to the laser diode 101 through the current mirror circuit which is constituted by the NMOS transistor 111 and the NMOS transistor 112. For this reason, in a case where the driving current to be supplied to the light emitting element 101 is stopped, if the gate voltage of the current mirror circuit becomes lower than a threshold voltage of the NMOS transistor 111, the NMOS transistor 111 comes to be in a non-conductive state. Consequently, since the route along which electric charges flow from the gate of the current mirror circuit to the ground potential is unavailable, discharge from the gate terminal of the current mirror circuit is delayed. For this reason, since a rise time of the current to be supplied to the laser diode 101 depends on a just-before off time, it is impossible to accurately control high-speed switching driving.
On the other hand, in the light emitting element driving circuit illustrated in FIG. 10, in a case where the driving current to the laser diode 101 is stopped, the NMOS transistor 116 comes to be in a conductive state, whereby the second current I2 flows in the NMOS transistor 116. For this reason, the gate voltage of the NMOS transistor 115 is determined based on the on resistance of the NMOS transistor 116. At this time, if the gate voltage of the NMOS transistor 115 exceeds a threshold voltage, the current is supplied from the NMOS transistor 115 to the laser diode 101. In recent years, the threshold current of the laser diode has reduced up to several milliamperes (mA). Consequently, in case of off controlling the laser diode, it is necessary to lower the on resistance of the NMOS transistor 116 so that the laser diode 101 does not emit light. In particular, in a case where the threshold voltage of the NMOS transistor 115 is low, a leakage current between the source and the drain is large, and the second current I2 is large, then it is necessary to enlarge the size of the W/L of the NMOS transistor 116.